Pressure measurement method

ABSTRACT

A method of determining the pressure applied to a limb by an inflatable garment by placing a flexible pressurizable cuff between the garment and the limb to receive the compressive garment forces and then determining the pressure in the cuff due to those compressive forces alone. The cuff pressure component due to compression of the garment is determined by extrapolating from a plot of cuff pressure versus bias pressure. The bias pressure is required to obtain an accurate reading of cuff pressure and is eliminated as a component in determining the garment pressure by means of the extrapolation. A preferred transducer for converting cuff pressure to electrical signals employs a manometer tube extending between two plates of a capacitor.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 164,136, filed6/30/80, now U.S. Pat. No. 4,331,133.

CROSS REFERENCE TO RELATED APPLICATION

This invention relates to pressure measurement apparatus and methodsgenerally and particularly for use in determining the pressure appliedby an inflatable or elastic garment to an injured part of a patient'sbody.

Inflatable splints or other inflatable garments have become a convenientmeans for keeping broken bones immobile and for the treatment of otherconditions such as prevention of deep vein thrombosis, edema and venousulcers. The pressure applied to the injured area by the garment dependson the extent of inflation of the garment itself. If the pressure is toogreat, the blood circulation to the limb can be adversely affected andif the pressure in the garment is too little, the garment becomesineffective as a splint. It is therefore necessary that the pressureapplied by the garment be measured.

A known way of measuring the pressure in the inflatable garments hasbeen to measure it directly with measurement devices built into thegarment itself. Examples of such devices are shown in U.S. Pat. Nos.2,823,668; 2,699,165; 2,113,253; and 4,039,039.

For purposes of economy and easy storage many other inflatable garmentshave no built-in pressure measuring systems. These garments aremanufactured in a fully sealed condition, such that the subsequentapplication of a direct measuring device to the garment itself is notpractical. Accordingly, in the past, the judgment of the personinflating such garments had to be relied upon to determine the correctamount of pressure. Such judgment is not always adequate, such that thedesired result is not achieved because of inadequate pressure orcirculation is impaired and the patient suffers discomfort or furtherinjury because of too much pressure.

The extrapolation is performed in two ways. In one embodiment the biaspressure and total cuff pressure are converted to electrical signals andsupplied to the x and y inputs, respectively, of an x-y chart recorderwhile bias pressure is varied. This produces a plot of cuff pressureversus bias pressure, and the extrapolation is done graphically. Inanother embodiment the formula for total cuff pressure is determined andsolved for zero bias pressure by a suitable microcomputer.

The amount of bias pressure is estimated by measuring the amount of gasinjected into the cuff. Thus, an important feature is the provision ofapparatus for injecting known amounts of gas into the cuff and means formeasuring such amounts and producing an electrical signal representativethereof.

The accurate measurement of the total cuff compression is important indetermining the garment compressive forces, and, in keeping with anotheraspect of the invention, a pressure transducer is provided with anelectrical circuit having a reactive imedance element which produces aninput signal that varies proportionately with the impedance of theelement, and means for varying the impedance in accordance withvariations of pressure. A particularly advantageous feature of thepreferred embodiment of the transducer is that a manometer is used tovary the impedance.

DESCRIPTION OF THE DRAWINGS

The foregoing objects, features and advantages will be explained ingreater detail and further features and advantages will be made apparentfrom the following description of the preferred embodiment given withreference to the below described drawings and the claims.

In the drawings:

FIG. 1 is a schematic illustration of the pressure measurement apparatusof the present invention;

FIG. 2 is an enlarged sectional view of inflatable cuff of the apparatusof FIG. 1;

FIG. 3 is an illustrative graph of cuff pressure versus bias pressure;and

FIG. 4 is a schematic illustration of the preferred embodiment of thepressure transducer of the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the pressure measurement apparatus 10 is shown inpartial schematic view as being used to determine the compressive forcesapplied to a leg 11 by an inflatable garment 12. Garment 12 is shownsecured to leg 11 as it would appear in application for a broken leg. Aflexible, pressurizable member, or cuff 14, is wrapped around the ankleof leg 11 and between inflatable garment 12 and leg 11, as most clearlyseen in FIG. 2. Alternately, a smaller cuff 14 could be merely insertedbetween the leg and garment 12 and not wrapped all the way around theleg. A flexible input tube 16 connects the interior of cuff 14 with asyringe-like device 28. Another flexible tube 18 connects the interiorof cuff 14 to a pressure transducer 22 used to measure the totalpressure in cuff 14 and produce a representative signal on its output23.

Another transducer 38 produces another electrical signal on the output39 representative of the bias pressure introduced by device 28. The twoelectrical pressure signals from transducers 22 and 38 which areproperly conditioned with respect to gain, offset and linearity, areapplied to inputs 24 and 25 respectively of a recorder 26. Recorder 26either records the signals on paper for graphic extrapolation ortemporarily, electronically records the signals and calculates thepressure of garment 12, and displays it on a digital readout 27.

Device 28 is used to inject air or other gas into the cuff 14 to createa bias pressure therein. As seen in FIG. 1, it comprises a syringe-likedevice having a plunger 34 snugly mounted for sliding movement within acylinder 32. The plunger 34 is manually actuated by pushing or pulling aplunger handle 30 connected to plunger 34 by shaft 31. Movement ofplunger 34 toward connector 36 by a preselected amount infuses apreselected amount of air through a tube 16 and into cuff 14. A checkvalve 33 prevents return of air from cuff 14.

Transducer 38 comprises a potentiometer resistor 41 with one endconnected to ground and the other end connected to a suitable D.C.supply voltage +V. The output 39 is taken from the potentiometer tap 43which is mechanically connected to plunger 34 so that it slides acrossresistor 41 proportionately with movement of plunger 34 by means of asuitable linkage schematically illustrated by broken line 45. As theplunger 34 moves toward connector 36, a proportionately increasing D.C.signal is produced on output 39 and applied to recorder input 25. Thissignal is thus representative of the amount of air injected into cuff 14which is approximately proportioned to the increase in bias pressure inthe cuff due to injections of such amount of air.

Pressure transducer 22, on the other hand, produces an electrical signalon its output 23 having an amplitude which varies proportionately withand is representative of the total pressure in cuff 14. While a varietyof known types of transducers, such as a diaphragm-strain gauge typetransducer, may be used for this purpose, the transducer shown in FIG. 4and described below is preferred.

Another suitable form of transducer 22 of the present invention is oneidentical to transducer 38 and linked to a syringe-like device identicalto device 28, except with a spring located at 64 resisting movement ofplunger 34 away from connector 36. When using such a transducer, tube 18is attached to connector 36, so that the cuff pressure pushes theplunger away from the connector, and the proportional signal produced onthis potentiometer resistor is applied to input 24.

Recorder 26 receives both the bias pressure representative signal andthe cuff pressure representative signal and simultaneously records both,so that there is a cuff pressure recorded for each recording of biaspressure. In accordance with one embodiment, the recorder is an ordinaryx-y chart recorder which records the two signals on a strip of paper inthe form of a plot, or graph, of cuff pressure versus bias pressure ofthe type illustrated in FIG. 3. As will be described below, from such aplot the cuff pressure component due to the compression of the garment12 is determined by graphic extrapolation. Alternately, the signals maybe temporarily stored electronically and the extrapolation calculated bya suitable computer, in which case the resultant garment pressure isdisplayed on readout 27.

In using the measurement apparatus 10, the cuff 14 is inserted in adeflated form between inflatable garment 12 and leg 11, either before orafter garment 12 is inflated. The garment 12 is inflated to what isestimated to be the proper pressure. Device 28 is then actuated toinject cuff 14 with known amounts of air. Injection continues untilafter any voids between the cuff and garment have been eliminated andair spaces 20 have been fully formed, so that accurate cuff pressuremeasurements may be made. During injection the bias pressurerepresentative signal and the resulting increasing cuff pressure signalare generated and recorded.

Referring to FIG. 3, an illustration of the variation of cuff pressurewith increases on bias pressure is shown. Units on the graph are inmillimeters of mercury. When there is no air in the cuff, i.e., whenbias pressure is zero, the indicated cuff compression is zero eventhough compression is being applied by the garment 12. This is becausethere is no air pressure in the cuff for the transducer to measure.After an amount of air has been injected into the cuff, such that thereis bias air pressure in the cuff, the compressive forces of the garment12 will operate on the cuff to reduce its volume and increase thepressure.

At the beginning of injecton, when there is an insufficient amount ofair to prevent partial collapse of the cuff and to eliminate voids, thepressure transducer records rapid increases in cuff pressure forrelatively small increases in bias pressure. This is seen in FIG. 3 forbias pressure in the range of zero to ten millimeters of mercury. Inthis range of bias pressure it is believed that the cuff pressure is notan accurate measure of the garment pressure because of the voids andpartial collapse, as described above.

However, as the bias pressure continues to increase beyond this point,the cuff pressure increases more slowly and linearly. In this range ofbias pressure, the cuff pressure has two components. This firstcomponent is, of course, due to the injected air and is the biaspressure. As more air is injected, the cuff pressure will naturallyincrease. The other component of cuff pressure is due to the compressiveforce applied to the garment against the cuff which is proportional tothe garment pressure. As compression increases, cuff volume decreasesand cuff pressure increases. With the garment pressure constant, as isthe case illustrated in FIG. 3, it is found that the cuff pressureincreases more gradually but in a linear fashion.

The bias pressure of the cuff against the garment is believed to alterthe garment pressure in addition to changing the cuff pressure. Thus, inorder to obtain an accurate measure of the garment pressure when thereis no bias pressure, the linear region of the plot is extrapolateddownward to zero bias pressure to determine the theoretical cuffpressure at zero bias pressure. If the cuff pressure and bias pressureare plotted on paper by a chart recorder, the extrapolation can be donegraphically as illustrated by broken-line 65 in FIG. 3 which intersectsthe y or cuff pressure axis at approximately 58 millimeters of mercury.

Alternatively, the extrapolation can be done analytically by a suitablecomputer. The more gradually sloped linear portion of the plot spacedfrom the origin, i.e., the portion in the range of twenty to fiftymillimeters of mercury, is described by the equation Pc=mPb+Po where Pcis the cuff pressure, m is the slope of the linear plot, Pb is the biaspressure and Po is the theoretical cuff pressure existing when Pb equalszero. Thus, the computer need merely determine the value of m, selectany point on the linear curve for values of Pc and Pb and solve for Po.

In either event, after the cuff pressure component due to the garmentpressure is determined, the cuff is deflated by means of a suitablerelease valve. If adjustments to the garment pressure are indicated, thecuff is kept in place for a new measurement after the garment pressureis adjusted. Once the desired garment pressure is achieved, the cuff isremoved, and the apparatus may be used to measure the pressure ofanother garment.

Referring now to FIG. 4, a preferred form of a pressure transducer foruse as pressure transducer 22 of FIG. 1 is seen to include a manometer50 having a manometer tube 62 with manometer fluid 63 therein extendingbetween a pair of plates 54 of a capacitor 55. The other end of the tubeis connected to a fluid basin 60 which is in pneumatic communicationthrough tube 18 with the pressure cuff 14 (not shown).

The manometer fluid level rises with increases of cuff pressure andfalls with decreases in cuff pressure. A fluid is selected such asmercury, which has a dielectric constant different from what of whatevermatter, such as air, is otherwise located between the plate 54.Accordingly, the capacitance of the capacitor 55 changes in accordancewith movement of the manometer fluid level and cuff pressure. The use ofthe manometer is particularly advantageous, since it is the standardpressure measurement reference and is inherently linear and easy to zeroand calibrate. The capacitor is connected to a capacitive bridge,oscillator or other circuit 56 which produces a resulting electricalsignal on its output 57 representative of the cuff pressure. Thissignal, in turn, is connected to recorder 26.

The foregoing detailed description is given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.For example, while a capacitor is employed in connection with themanometer of the transducer of FIG. 4, it is contemplated that the sametechnique could be used with an inductance coil.

I claim:
 1. A method of measuring the compression applied to a portionof a body by a resilient inflatable garment, comprising the stepsof:inserting a flexible pressurizable member between the body portionand the garment to receive the compression applied by the garment; anddetermining the pressure in said member due to the compression, saidpressure being approximately equal to the compression applied to thebody portion.
 2. The method of claim 1 in which said step of determiningthe pressure includes the steps of:injecting the pressurizable memberwith gas to produce varying amounts of bias pressure therein; measuringthe varying amounts of bias pressure; measuring the total pressurewithin the pressurizable member for said varying amounts of biaspressure, said total pressure varying with varying amounts of biaspressure; and extrapolating to zero bias pressure the variation of totalpressure relative to the varying amounts of bias pressure to determinethe total pressure when there is an absence of any bias pressure.
 3. Themethod of claim 2 in which said step of measuring the bias pressureincludes the steps ofmeasuring the amounts of gas injected, andestimating the bias pressure resulting in the pressurizable member dueto injection of said amounts.
 4. The method of claim 3 in which said gasis injected into said pressurizable member by means of a plunger movedthrough a cylinder and said step of measuring the amounts of gasinjected includes the step of measuring said plunger movement.
 5. Themethod of claim 2 in which said step of extrapolating includes the stepsofplotting the total pressure versus the bias pressure for varyingamounts of bias pressure on an x-y graph to develop an approximatelinear portion of the plot spaced from the origin, graphically extendingsaid linear portion to the zero bias pressure coordinate, and readingthe total pressure at the intersection of said extended portion with theaxis of the graph along which the total pressure is indicated.
 6. Themethod of claim 5 in which said step of plotting includes the stepsofdeveloping an electrical signal representative of the total pressure,developing an electrical signal representative of the bias pressure, andsimultaneously feeding both electrical signals to the x and y inputs ofan x-y chart recorder.
 7. The method of claim 2 in which said step ofextrapolating includes the steps ofdetermining the formula for totalpressure as a function of bias pressure for a range of bias pressureswhich excludes zero bias pressure and for which total pressure can beaccurately measured, and then solving said formula for zero biaspressure.